In the event manager's registerFd_, we find: registerFd_ :: EventManager -> IOCallback -> Fd -> Event -> Lifetime -> IO (FdKey, Bool) registerFd_ mgr@(EventManager{..}) cb !fd !evs lt = do ... <- ... (modify,ok) <- withMVar (callbackTableVar mgr fd) $ \tbl -> do ... <- ... -- this simulates behavior of old IO manager: -- i.e. just call the callback if the registration fails. when (not ok) (cb reg evs) return (reg,modify) A comment and a question. Comment: registerFd_ is only ever called in contexts where exceptions are masked, so withMVar is doing some needless mask/restore. Question: why do we immidiately call the callback if event registration fails? This means that if event registration fails during something like `threadWaitRead`, the end result would be that `threadWaitRead` would just return immidiately. That doesn't seem good. -- -Andrew Thaddeus Martin